A memory cell of a dynamic random access memory (DRAM) is composed of a metal oxide semiconductor (MOS) transistor and a capacitor. The capacitor is generally designed as either a stack capacitor stacked on a substrate or a deep trench capacitor buried within a substrate.
A common deep trench capacitor is a small three-dimensional device formed by etching a trench into a semiconductor substrate of the device. After trench etching, a doped region may be formed in the lower portion around and below the trench, which serves as a buried plate electrode of a trench capacitor. A dielectric layer may be formed over the buried plate electrode in the trench. The dielectric layer serves as an insulating layer between the electrodes of the trench capacitor. The trench is filled, for example, with conductive polycrystalline silicon (poly-Si), which serves as an upper electrode of the trench capacitor.
In order to increase capacitance of the trench capacitor, a dual deep trench capacitor is provided. The dual deep trench capacitor has four layers composed of a lower conductive layer, an upper conductive layer, and two dielectric layers which are respectively disposed beneath the lower conductive layer and between the lower conductive layer and the upper conductive layer. After forming the four layers, an inter-layer dielectric (ILD) layer is formed covering the dual deep trench capacitor. However, topology variations from the formation of the four layers in the trench and on the top of the semiconductor substrate result in an uneven surface of the ILD layer. Therefore, semiconductor planarization techniques, such as chemical mechanical polishing (CMP), have been utilized in smoothing the topology variations of the ILD layer. Nevertheless, the ILD layer has a relatively low thickness that may be polished over during the polishing process and short the upper conductive layer and any electronic component disposed on the ILD layer. Accordingly, improvements in structures and methods of forming the dual trench capacitor continue to be sought.